US President Donald Trump said on Friday he was ordering American companies to look at ways to close their operations in China and make more of their products in the US instead, a rhetorical strike at Beijing as trade tensions mounted.

Trump cannot legally compel US companies to abandon China immediately and he gave no detail on how he might proceed with any such order, although he said he would be offering a response later on Friday to tariffs on American products announced by China earlier in the day.

The US dollar rose sharply against the Chinese yuan and US stock markets fell on Trump’s latest salvo against China.

“Our great American companies are hereby ordered to immediately start looking for an alternative to China, including bringing your companies HOME and making your products in the USA," Trump wrote on Twitter. “We don’t need China and, frankly, would be far better off without them."

For many products sold in the US, there are few alternatives to Chinese production, and shifting production for goods produced there could take years and be expensive.

Trump’s comments came on a day Beijing announced $75 billion of new tariffs on US goods, including soybeans, cars and oil. This was in retaliation to the US president’s latest planned levies on Chinese imports.

The move takes aim at the heart of Trump’s political support—factories and farms across the Midwest and South at a time when the US economy is showing signs of slowing down.

Some of the countermeasures will take effect starting 1 September, while the rest will come into effect from 15 December, according to the announcement from China’s finance ministry.

This mirrors the timetable the US has laid out for 10% tariffs on nearly $300 billion of Chinese shipments. An extra 5% tariff will be put on American soybeans and crude-oil imports starting next month. The resumption of a suspended extra 25% duty on US cars will resume 15 December, with another 10% on top for some vehicles. With existing general duties on autos taken into account, the total tariff charged on US-made cars would be as high as 50%.

The developments rekindled concerns about the world’s two largest economies and a global growth outlook that’s already looking shaky.

Last week, Trump backed off his 1 September deadline for 10% tariffs on remaining Chinese imports, delaying duties on cellphones, laptops and other consumer goods.

This story has been published from a wire agency feed without modifications to the text. Only the headline has been changed.